From German patent document DE 41 32 759, there is known a grass conditioning device in which a plurality of planetary cylinders are arranged about a central cylinder, the planetary cylinders having a substantially smaller diameter than the central cylinder. In a harvester (grass mowing machine), this arrangement disadvantageously requires space on account of its height, whereby, for example, an installation space provided for a drive motor of the harvester is restricted. Moreover, this design requires unequal cylinder sizes, whereby the production, and in particular the spare parts storage, of the various cylinders is complicated and cost-intensive.